1. Field of the Invention
This invention relates to a portable type wireless communications unit using a battery as the power supply and a lighting control means thereof.
2. Description of Related Art
In the prior art, portable telephones such as cellular phones can be freely used in a wide variety of locations and are therefore extremely useful for business and other situations.
This portable telephone incorporates a secondary battery and connects the user to the nearest base station by means of wireless communication in the UHF band thus providing a comparatively wide range of movement. The audio signal of the telephone is digitally processed and transmitted with its time axis and amount of data compressed.
A portable telephone as described above has an outer appearance as shown for instance in FIG. 5.
In a portable telephone 10 shown in FIG. 5, a receiving antenna 11 is installed at the upper part of the case 10c. A speaker 12 and microphone 13 are installed at the top and respectively bottom of the front of the case 10c. A plurality of dial keys 14 and a plurality of function keys 15a through 15m are provided between the speaker 12 and microphone 13. A display 16 utilizing a liquid crystal display element is provided.
This liquid crystal display 16 shows information on the display such as used for instance in a telephone book function to search for telephone numbers, provides good visibility by means of a source of light such as a backlight, in which the display is illuminated from the rear. The light source for this backlight is an incandescent lamp or a light emitting diode.
The dial keys 14 and the function keys 15a through 15m each incorporate a light emitting diode for display illumination. These keys light up when the power is turned on and are easily visible even in dark locations.
The power supply for the portable telephone of the prior art is generally nickel-cadmium storage batteries or nickel-hydrogen storage batteries. Recently, however lithium-ion storage batteries having higher capacity and higher energy density are being used.
These lithium-ion batteries have a nominal voltage of 3.6 volts which is three times higher than the 1.2 volts (nominal) of nickel-cadmium or nickel-hydrogen batteries. The number of recharge cycles is also 1200 times that of nickel-cadmium or nickel-hydrogen batteries yielding a battery life more than twice as long as the 500 hours of nickel-cadmium or nickel-hydrogen batteries.
Discharge characteristics of lithium-ion batteries as shown in FIG. 4 have a steeper slope than nickel-cadmium or nickel-hydrogen batteries as can be seen in FIG. 4 along with a monotone decrease characteristic.
The compactness and light weight of these portable telephones places limits on the capacity of their internalized secondary batteries so that these portable telephones usually incorporate a battery voltage sensor circuit. In the final stages of secondary battery life it becomes difficult to supply the required stable voltage Vst for the circuits that provide a load on the battery. This is the so-called "LOW BATTERY" condition and an alarm is issued just before this stage is reached. The user responds to this alarm by charging or replacing the secondary battery.
These portable telephones connect many users with their respective parties while utilizing the same frequency band. In order to prevent each pair of callers from interfering with other callers, a system called time-division multiple access (TDMA) is employed which segments the calling time per each user to allow common use on the same frequency.
In this TMDA system, transmit and receive is performed by time sharing or time division. Restated, the audio data is placed on a time base and the quantity of data compressed, and the transmit data (base band signal) is then generated in bursts. The transmit signal modified by this transmit data is assigned to a transmit slot of specified timing and then sent while the receive signal from the other party is received by a receive slot which is separate from the transmit slot.
An idle slot is present between the receive slot and the transmit slot as shown in FIG. 6A. Receive and transmit on the portable telephone is performed repeatedly while segmented according to time. The time lengths for the transmit slots Tr, Ti and Tt are for instance set equally and the repeat interval is set for instance to 20 milliseconds.
The secondary battery internalized in the portable telephone puts limits on its own capacity due to the compactness and light weight of the portable telephone. During transmit, when power consumption is greatest, a voltage drop occurs in the secondary battery due to internal resistance.
First of all, operation prior to the time point tob is illustrated in FIG. 6B. In the interval where the power for the background light is "OFF" the power consumption is comparatively small at the receive slots and idle slots, and as shown in FIG. 6C, the secondary battery voltage becomes Vn. The secondary voltage at the transmit slot drops to: EQU Vt=Vn-.DELTA.Vt
Here, .DELTA.Vt is the internal voltage drop versus power consumption during transmit.
At the time point tob within the receive or the idle slot in FIG. 6B, when the backlight power source for the liquid crystal display 16 changes from `OFF` to `ON` all slots from the time point tob have an internal voltage drop .DELTA.Vb in response to the power consumption from the backlight power source as shown in FIG. 6 when the backlight power source is at `ON` the secondary battery terminal voltage at the transmit slot drops to EQU Vtb=Vn-.DELTA.Vt-.DELTA.Vb
This current consumption may reach for instance one ampere during transmit. Current consumption may climb another 50 to 60 milliamps due to lighting of the backlight for the liquid crystal display 16.
As can be seen therefore in the portable telephone of the prior art, when the backlight light source is at the ON interval during the transmit slot, an internal voltage drop .DELTA.Vb is added to the internal power consumption .DELTA.Vt during transmit due to power consumption by the backlight, so that the voltage at the terminals of the secondary battery is even lower than when the backlight source is in the OFF interval.
During initial discharge of the secondary battery as shown in FIG. 4, the voltage required at the battery terminals is sufficiently higher than Vst so that even if lighting of the backlight coincides with operation of the transmit circuit, the voltage at the secondary battery terminals will not drop down to the threshold voltage where the battery voltage sensor will trigger a "LOW BATTERY" alarm.
However in the final stages of secondary battery discharge when the lighting such as for a backlight coincides with transmit circuit operation, the voltage at the secondary battery terminals will drop down to this threshold voltage, and the battery voltage sensor will mistake this for a "LOW BATTERY" condition.
Further, as shown in FIG. 4, when using a secondary battery having the monotone decrease characteristic, portable telephones of the prior art have the problem of a difference in backlight brightness depending on whether the secondary battery is in the initial or final stages of discharge.